Fast pyrolysis is a high temperature process (350 to 800° C.) in which a biologically based feedstock is rapidly heated in the absence of air and vaporizes into a product gas stream. Although the smoke-containing gas stream that is produced comprises both combustible and inert components, of greater interest is a liquid product that is usually condensable therefrom. One common oily liquid product that may be condensed from the product gas stream is dark brown in colour and may be burned as a bio-oil having a heating value of about half that of conventional fuel oil. Other liquid products can contain, for example, flavour enhancing constituents or even pharmaceutically active compounds. The char and ash remaining after pyrolysis can also have value as fuel or chemical feedstocks.
The characteristics of the liquid product depend upon the biological feedstock being used and the processing conditions. In fast pyrolysis, reactions typically take place in less than 3 seconds and are carefully controlled to provide the desired product characteristics. Fluid bed reactors are often used to provide rapid heat and mass transfer and inert solid carriers (eg: silica sand) are sometimes used as an aid in heat transfer and biomass processing. These fluidized bed fast pyrolysis processes are well documented and have been used in many different applications for several decades. See, for example, Bridgwater, A. V.; Peacocke, G. V. C. “Fast pyrolysis processes for biomass” Renewable and Sustainable Energy Reviews, v. 4 (2000), pp. 1-73, for a review of prior art fluid bed fast pyrolysis processes developed over the last 20 years.
Low density agricultural biomass (eg: straw, stalks, husks, chaff, bark, wood chips, etc.) is typically widely dispersed within a given geographical area, difficult to collect and expensive to transport economically. In the fast pyrolysis of low density agricultural biomass, a common problem that makes the process uneconomical is the prohibitive cost of transporting these bulky materials to a central processing site. Application of fluid bed fast pyrolysis technology to such materials therefore relies upon a significant reduction or elimination of these material transportation costs. It would be desirable to provide “self contained” fast pyrolysis process equipment that is compact, mobile and has the ability to be set up and operated close to the source of the feed materials. To make it easier for farmers and workmen, particularly in the developing world, to take advantage of such mobile equipment, it would also be desirable that the equipment be simple to operate and flexible in terms of the choice of fuel source. The process employed in using the equipment should be forgiving in terms of particle size and biomass moisture content. The process should also take advantage of rapid heat transfer and short solids residence time to reduce vessel size and increase throughput.
U.S. Pat. Nos. 5,728,271 and 5,853,548 disclose a thermolysis process for liquefaction of biomass solids employing separate combustion and fluidized bed reaction vessels. The process takes place at low temperature and employs long residence times. This process is therefore not suitable for mobile implementation.
U.S. Pat. Nos. 6,048,374 and 6,830,597 disclose a pyrolysis process for gasifying and/or liquefying biomass that employs combined fluidized bed and combustion reactors. The fluidized bed reactor is in the form of a tube and biomass moves downwardly through the tube. The combustion chamber partially surrounds the reactor tube and heat from the combustion chamber passes through the reactor wall and directly heats the biomass within the interior reactor tube. The reactor is in communication with the combustion chamber through a bottom opening that allows char to be consumed in the combustion chamber as fuel. The reactor does not contain any fluidizable media and no fluidized bed is employed, which limits heat and mass transfer within the reactor.
U.S. Pat. Nos. 5,792,340 and 5,961,786 disclose fast pyrolysis processes for the production of a liquid product containing flavour enhancing constituents. The processes employ a fluidized bed transport reactor that utilizes gas velocities high enough to cause biomass and an inert heat carrying media to move upwardly within the reactor. The biomass and inert carrier are separated from the product gas stream by a cyclone and re-introduced into the bottom of the bed. The combustion chamber is separate from the reactor. Although this configuration leads to rapid mass transfer, it requires careful control of particle size, is complicated to operate and is not well-suited to mobile implementation due to its size and energy requirements.
U.S. Pat. No. 4,917,028 discloses a pyrolysis reactor that includes an interior drying zone where biomass moves downwardly, a lower combustion zone where heat is added and where biomass particles are conveyed upwardly, and an outer pyrolysis zone where biomass is fluidized and is deflected back towards the interior drying zone to rain down upon the biomass therein. This reactor does not employ lift tubes but rather circulates the entire bed between the combustion chamber and pyrolysis reactor; as a result, there is both fluid and solid communication between the zones, which makes it difficult to precisely control process conditions and achieve the desired product characteristics.
To date, conventional fast pyrolysis processes employ multiple vessels, are complex to operate, are inflexible and/or are not suitable for mobile operation. Compact systems that combine the combustion chamber and pyrolysis reactor in a single vessel usually involve mixing of fluids and/or solids between the two portions of the vessel; this leads to contamination and/or destruction of the most valuable condensable liquid products in the product gas.
The need therefore exists for an improved apparatus for pyrolysis of low density agricultural biomass and a process for use thereof.